Method for the heat treatment of alloys



'ture is not amenable to (heat) treatment.

METHOD FOR THE HEAT TREATMENT OF ALLOYS Erich Fitzer, Meitingen, near Augsburg, Germany No Drawing. Application July 17, 1957 Serial'No. 672,350

2-Claims. c1.14s-.-1s.1

This invention relates to a method for the heat treatment of alloys.

Nickel-aluminum alloys have already been proposed for use at high temperatures. In particular, the surprisingly' high melting point of the intermetallic compound NiAl permits one to expect good high temperature properties on the part of such alloys. It has now been possible to discover that with respect to the resistance to scaling, the phase which is richest in aluminum, namely Y NiAl is not the compound of the system which is most resistant to oxidation. NiAl scales with the formation of A1 and MO which occur alongside of eachother. The pure Ni Al phase which is richer in nickel is also not particularly resistant to scaling, although its resistance is greater than that of NiAl. Surprisingly enough the heterogeneous NiA1/Ni Al alloy is most resistant to scaling. In cast condition such alloys cannot be used for all practical purpose due to the fact that the cast struc- One way out in this connection is afforded by powder metallurgy. The production by powder metallurgy of NiAl alloys is actually possible. Itv has now been found that bodies having the greatest sinter densities and strengths are produced if'completely oxide-firee'powders are used and if the sintering is effected in an atmosphere which is entirely free of oxygen and hydrogen.

By such sintering treatments, there are obtained sintered bodies having about 90 percent space filling, which constitutes a good resultin the case of these very sensitive substances. These bodies have pores of very fine but rather irregular shape.

NiAl alloys produced in this manner by powder metallurgy do not in the slightest exhibit the brittleness of the cast substances. On the contrary, even at only slightly increased temperature, they are subject to strong plastic deformation and thus entirely unsuitable as flow-resistant materials for high temperatures.

, W ee PM 0 It has now been found that the flow behavior of such sintered alloys can be decidedly improved by subjecting them after the sintering in a non-oxidizing atmosphere to a heat treatment in an oxidizing atmosphere at temperatures above the temperature of use but below 1370 C.

and preferably between 950 C. and 1300 C. Surprisingly, it has now been found that the high temperature strength itself can also be decidedly'increased by treatment in accordance with the invention. This behavior is surprising inasmuch as due to the oxidation there was to be expected either the formation of a protective layer on the surface or else, in case of internal oxidation, at

' destruction of the structure. Actually such structural destruction does occur if the NiAl alloys subjected to oxidation contain 20 percent to 30 percent pore volume.

Sintered bodies on the other hand which have 5 to 15 percent pores in finely divided uniform size are excellen-tly well suited with respect to the compacting which can be obtained. This compacting does not occur upon the oxidizing heat treatment of molten specimens. Thus there cannot be concerned a so-called internal oxidation by absorption of dissolved oxygen. Systematic examinations make it appear probable that the NiAl-phase firaction-is converted by .oxidation partly into. A1 0,, while the balance forms Ni Al. The method in accordance with the invention is accordingly limited to alloys which contain NiAl phase in addition to Ni Al phase and, therefore, to alloys having Ni contents of 65 percent to 87 the flexural breaking strengths 6 in kg./mm. and the deflections E in mm. in un heat-ltreated condition gave the following values:

Temperature in 0. ,a v e uses The values were improved as follows by the treatment in accordance with the invention:

Temperature ln G. 6

(2) A sinter alloy containing percent Ni, balance Aland 7 percent pore volume, upon fatigue bending with a load of 20 kg./mm. at 500 C. shows, during thefirst L 50 hours an increase of the deflection from 0.4 to 0.8 mm. The same specimen when heat treated in accordance with the invention for 100 hours at 1150 C. shows a completely invariable deflection of 0.3 mm. under the same test conditions.

Upon fatigue loading at 600 C., the northeat-treated test piece broke after 20 hours while the heat treated piece showed an increase of the deflection of from 0.4 to 0.7 mm.

(3) A sinter alloy containing 70 percent Ni, -.balance 7 Al and 12 percent pore volume exhibits the following improvements after the oxidizing heat treatment for hours at 1000 C.: At a test temperature of 950 C., the bending strength increases from 15 kg./mm. wi 1.1 mm. deflection to 32 kg./mm.3 with 1.4 mm. deflection. I

The same improving effect is obtained if instead of Ni-Al complete alloys, compacts of nonmetallic or intermetallic compounds with such Ni--Al subjected to an oxidizing treatment.

What is claimed is: '1. A method of improving the flow behavior of nickel alloys are aluminum alloys having a nickel content from 65 to 87" percent, balance aluminum, said alloys having a pore volume ranging from 5 to 15 percent, comprising sintering said alloys in a non-oxidizing atmosphere and heat treating the sintered alloy in an oxidizing atmosphere at temperatures ranging from 950 to 1370" C.

2,936,255 I Patented Ma y lO, v}

n 4 v 2. A method of compacting and For increasing the high oxidizing atmosphere at a temperature ranging from 950 temperature strength of nickel aluminum alloys contain- C. to 137 0 C. ing from 65 to 87 percent of nickel, balance aluminum, References Cited in the file of this patent said alloys having a pore volume ranging from 5 tb 1 5 percent, comprising sintering said alloys in a nQn-oxidiz- 5 UNITED STATES PATENTS ing atmosphere and heat treating said, alloys in an 2,755,184 Turner et a1. July 17, 1956 

1. A METHOD OF IMPROVIDING THE FLOW BEHAVIOUR OF NICKEL ALUMINUIM ALLOYS HAVING A NICKEL CONTENT FROM 65 TO 87 PERCENT, BALANCE ALUMINUIM SAID ALLOYS HAVING A PORE VOLUME RANGING FROM 5 TO 15 PERCENT, COMPRISING SINTERING SAID ALLOYS IN A NON-OXIDIZING ATMOSPHERE AND HEAT TREATING THE SINTERED ALLOY IN AN OXIDIZING ATMOSPHERE AT TEMPERATURES RANGING FROM 950 TO 1370*C. 